The present invention relates to an electrical signal filter and method for manufacture of an electrical signal filter internal circuit board, and more specifically, relates to a electrical signal filter applicable to cable television, and to a method for the manufacture of an circuit board provided internally of said signal filter.
In recent years there has been the diffusion of cable television (hereinunder termed CATV). CATV has many channels and the viewer concludes a contract with a cable television broadcasting channel and the television signals of that channel are then transmitted to his/her household. Some of these many channels are such that they can be viewed by the payment of an additional charge. Normally, channels for which an additional charge is to be paid are provided with electrical signal filters (trap filters) in the cable of the CATV, so that the signals having a particular frequency are cut. The system is configured so that the channel for which said additional payment has been made is supplied to the household after this electrical signal filter has been removed.
A conventional electrical signal filter of this type is disclosed in U.S. Pat. No. 4,451,803.
This electrical signal filter comprises a single circuit board having a filter circuit, a housing (shield case) housing this filter circuit and a shield portion that magnetically insulates this circuit board inside the housing.
This circuit board is divided into two portions, each of which are provided with different filter circuits. In addition, each of the filter circuits is electrically connected to the circuit board. Furthermore, at one of the ends of this circuit board is disposed a connector for output, and to the other of the ends is provided an input terminal.
The housing comprises the three portions of a filter housing portion, a filter cap, and a connector for a cylindrical housing. The filter housing portion has the circuit board inserted into it and encased with a filter cap, with the outer periphery being covered by the cylindrical housing. When the circuit board is inserted into this filter housing, the connectors provided to the circuit board are brought into engagement with the connector housing portion that has been formed beforehand on the filter housing portion.
In addition, there are two shield members provided and the two filter circuits inside the housing are magnetically shielded. These magnetic shield members are configured so that they are directly fixed to each of the circuit boards.
However, in such a conventional electrical signal filter, there are many parts comprising the housing and so there is the problem of the efficiency of assembly being poor.
In the aforementioned electrical signal filter, the cylindrical housing is provided so that it surrounds the outer periphery of the filter housing portion. Thus, it is necessary to provide shielding portions at both ends of the filter housing portions and the cylindrical housing. The signals are liable to leak at the shielding portions. In addition, the shielding portions do not have strong mechanical strength, as compared with other portions. For the above reason, it is desirable to reduce the number of shielding portions as small as possible.
Conventionally, rubber O-rings are provided at the shielding poritions, so that the shielding portions are protected from rain, dust and the like. However, the rubber O-rings deteriorate with age, which are prone to incorrect installation and damage during installation, and may accidentally be left out of the final assembled unit in the manufacturing process (and this cannot be verified since the O-rings are internal to the unit and not visible in the final product).
Not only this, as has been described earlier, when the housing is assembled, the connectors provided to the circuit board are brought into engagement with the connector housing portion that has been formed beforehand on the filter housing portion and these connectors have a cylindrical shape and are not provided with a mechanism whereby they can be fixed to the circuit board. Because of this, when the connectors are soldered to the circuit board, it is difficult to position the connectors at the required position, and this causes the problem of making the soldering work more difficult. Furthermore, if the positioning to the circuit board is not performed accurately, then there is the additional problem that the connectors will not be able to engage properly with the connector housing portion.
Furthermore, each of the shield members comprises a signal circular-shaped plate and is provided independently and separately above the circuit board and so it is difficult to solder each of the shield members and the filter housing portion when one has been encased, thereby to consequently cause the problem of there not being sufficient shielding performance.
In addition, a circuit board is provided to the inside of the electrical signal filter as has been described above but there are also the following problems when this circuit board is manufactured. The following is a description of the method of manufacture of a conventional circuit board of this type and the problems associated with the manufacture of a conventional circuit board of this type.
In general, circuit boards that have an electrical signal filter incorporated into them have a relatively small shape and because of this, many of them are manufactured from a large mother board of the required shape. One of the methods of manufacturing a plural number of circuit boards from this mother board is known as the pushback method.
This pushback method involves punching circuit boards of required shape by general press processing to the mother board prior to providing the electronic elements to the circuit board, and then returning these punched circuit boards to the mother board.
The following is a description of the method of manufacture of a circuit board by the conventional pushback method, with reference to FIG. 1 through FIG. 4.
FIG. 1 shows a mother board 1. Press punch processing is first performed with respect to this mother board 1 and circuit boards 2 of the predetermined and required shape are punched out. When this press punch processing is performed, the opening of insertion holes and the like for the electronic elements are also implemented at the same time.
Following this, the circuit boards 2 that have been punched out are again returned to the mother board 1. FIG. 2 shows the status where the circuit boards 2 that have been punched out are again returned to the mother board 1. The reason for returning the circuit boards 2 that have been punched out, back to the mother board 1 is that in the process for the provision of the electronic elements and the like and which is performed as the following process, it is easier if this work is performed with respect to the large mother board 1 rather than with respect to the small circuit boards 2 and this, coupled with the fact that it is possible to perform batch processing with respect to many circuit boards 2 enables the productivity to be increased.
As has been described above, when the circuit boards 2 are returned to the mother board 1, stress release holes 3 are punched as shown in FIG. 3. When the circuit boards 2 are punched from the mother board 1, burrs are created at the positions of the cuts and when the circuit boards 2 are returned to the mother board 1, these burrs enter between the circuit boards 2 and the mother board 1 to create stresses in the circuit boards 2 and warping may occur in the circuit boards 2 when these stresses are large. This stress and the occurrence of warping adversely affects the soldering and the following process for the provision of electronic elements and contributes to the lowering of the reliability of the product. Accordingly, the forming of the stress release holes 3 prevents the generation of this stress and the consequent warping.
When the stress release holes 3 are formed, the required processing for the provision of the electronic elements is implemented with respect to each of the circuit boards 2 that are monolithic with the mother board 1. FIG. 4 shows the status where this required processing has been completed.
Following this, when the processes described above have been completed, each of the circuit boards 2 is taken from the mother board 1 and when each of the circuit boards 2 is taken from the mother board, the conventional method has been to press the circuit boards 2 to remove them from the mother board 1.
In the method described above for the manufacture of a circuit board by the pushback method, it is desirable that the circuit boards 2 be firmly held to the mother board 1 when the electronic elements and the like are being provided. If this holding force is small, then there is the fear that the circuit boards 2 will become separated from the mother board 1 in the processes for the provision of the electronic elements and the like, as has been described above, to therefore create an undesirable situation for processing.
On the other hand, when the circuit boards 2 are firmly held onto the mother board 1, it becomes more difficult to take the circuit boards 2 off of the mother board 1 and the task of taking them off becomes more difficult. Because of this, when the stress release holes 3 are formed as described above, board removal holes are also formed so that the task of removing the circuit boards 2 from the mother board 1 is facilitated. However, when these board removal holes are also formed along with the stress release holes 3, the strength by which the mother board 1 holds the circuit boards 2 is reduced and there is the fear that the circuit boards 2 will fall off of the mother board 1 during the process for the provision of the electronic elements and the like.
In such a conventional manufacturing method, it is necessary to have a strong holding force in the provision of the electronic elements and the like and it is also desirable that there be a weak holding force when the circuit boards 2 are removed. There are therefore these mutually contradictory requirements and there is the problem that it is not possible to satisfy both of them at once.